This invention relates to hydrostatic power branching transmissions, and more particularly to a hydrostatic power branching transmission that is efficient, uncomplicated, reliable and practical.
A large variety of variable speed drive mechanisms are described in the literature or are currently commercially available. The mechanisms find application in fields as diverse as computers, machine tools, recreational vehicles, construction equipment, and automobiles. They all share the basic function of converting a rotational speed and torque at the input shaft to a variable speed and torque at the output shaft.
The motor vehicle is an ideal application for an infinitely variable speed drive mechanism because of the improved economy that can be obtained by running the vehicle's prime mover, such as an internal combustion engine at, or nearly at, its optimum operating point. Moreover, the potential market is enormous: it has been estimated that the annual world-wide market for automotive transmissions in the 15 years before 2005 will be in the vicinity of 47 million vehicles.
Although numerous infinitely variable transmissions and continuously variable transmissions have been proposed for automotive application, none has proven completely satisfactory. Traction devices have been unable to demonstrate acceptable life at the power levels required and the transient torque conditions occurring in a normal automotive driving cycle.
Rubber belt variator transmissions similar to snowmobile transmissions have found limited application in special automotive areas, such as the mini cars produced in Europe. One auto maker produces a compact car which uses a DAF belt variator transmission. Life and efficiency for these devices are marginal at best, even in light vehicles with engine power on the order of only ten to fifteen horsepower. The most common application for this type of transmission is the snowmobile, where component life is not expected to exceed 100 hours.
A metal belt variator transmission, known as the "Van Doorne" transmission, has received considerable publicity in the automotive press. This transmission is unique in that the power is transmitted by compression through the metal belt segments rather than by tension in the belt which is the usual method. Several production automobiles were introduced in Europe with the Van Doorne transmission in 1987, but the applications reported to-date have been limited in size to vehicles with engines of no more than 1.6 liters. The performance of vehicles equipped with the Van Doorne transmission is comparable to those equipped with conventional automatic transmissions despite the relatively poor 91% efficiency of the transmission. This is believed to be due to the efficiencies gained by operating the vehicle engine at its optimum operating point for much of the driving cycle.
Hydrostatic transmissions have existed for years and have been developed to a high degree of sophistication. These devices are in wide use in agriculture and construction equipment, mining and other off-the-road vehicles, and in small garden tractors. A conventional hydrostatic transmission is comprised of two principal elements: a hydraulic pump driven by the prime mover, and a hydraulic motor powered by hydraulic fluid pressurized by the pump for driving the load. Either or both of these elements may be variable displacement to achieve the variable gear ratio of the transmission. Regardless of the configuration selected, the overall system efficiency can be no better than the product of the efficiencies of the individual elements. For example, if both the pump and motor are 95% efficient, the hydrostatic unit cannot achieve efficiency greater than (0.95.times.0.95)=90% and in practice it is usually less than this because of flow losses in the hydraulic lines coupling the two elements. This efficiency does not compare favorably with conventional automatic transmissions which are capable of operation at steady state efficiency levels on the order of 97% to 98% with torque converter lock-up. Moreover, hydrostatic transmissions tend to be noisy, especially at the higher gear ratios where flow rate is greatest.
The integrated hydrostatic transmission is a step in the right direction. It combines the motor and pump in one unit to minimize fluid flow losses. However, none of the hydrostatic transmissions marketed to-date overcome the condition which degrades their efficiency and contribute to their noisiness, namely, the peak power rating of the transmission is attained at maximum pressure and flow. As a consequence, hydraulic losses associated with pressure, which consist of leakage and fluid compression and expansion will be highest at maximum power throughput. Also, viscous flow losses which are proportional to fluid velocities are greatest at peak power/speed when the flow and pressure are at their highest levels.
The fact that hydrostatic transmissions have not been introduced into production for automotive use is probably due to three main reasons: 1) high cost, 2) noise, and (3) poor efficiency. However, modern production techniques have been developed that would make it possible to produce a hydrostatic transmission designed specifically for automotive application at a cost approximately comparable to that of a conventional automatic transmission. The second and third factors, namely, noise and efficiency, have been the key factors discouraging adoption of a hydrostatic transmission by the automotive industry.
One effort to overcome some of the disadvantages of the conventional hydrostatic transmission is the power branching transmission. An early example of such a transmission is shown in U.S. Pat. No. 3,175,363 to Hans Molly. The power branching transmission was intended to reduce the fluid flow losses associated with the hydrostatic transmission, particularly as the transmission ratio moves toward unity, by mechanically transmitting a portion of the input power to the output shaft. Since the proportion of mechanically transmitted power increases to 100% at a 1:1 transmission ratio, the hydraulic losses are potentially much less in a power branching transmission.
Unfortunately, attempts to commercialize the power branching hydrostatic transmission have been unsuccessful, probably because of the great complexity of the system which would compromise performance and increase cost to an uncompetitive level versus the conventional transmission. Also, the prior art power branching transmissions have not been able to achieve a dynamic balance of the rotating elements which would be a serious shortcoming since substantial vibration levels at operating speed would not be acceptable.
Thus, the transmission art has long needed a hydrostatic transmission that incorporates the advantages of the integrated hydrostatic transmission while markedly reducing the losses associated with the conventional hydrostatic transmission.